Amphetamines, new psychoactive drugs and the monoamine transporter cycle

Abstract

In monoaminergic neurons, the vesicular transporters and the plasma membrane transporters operate in a relay. Amphetamine and its congeners target this relay to elicit their actions: most amphetamines are substrates, which pervert the relay to elicit efflux of monoamines into the synaptic cleft. However, some amphetamines act as transporter inhibitors. Both compound classes elicit profound psychostimulant effects, which render them liable to recreational abuse. Currently, a surge of new psychoactive substances occurs on a global scale. Chemists bypass drug bans by ingenuous structural variations, resulting in a rich pharmacology. A credible transport model must account for their distinct mode of action and link this to subtle differences in activity and undesired, potentially deleterious effects.

Keywords: addiction; amphetamine; monoamine transporter; psychostimulant; regulation; reverse transport.

Figure I

Chemical structures of amphetamines.

Figure I

The N- and C-termini of DAT and their regulatory inputs. The figure was obtained and modified with permission from James Foster and Roxanne Vaughan [64]. The N- and the C-termini of eukaryotic SLC6 transporters have >60 and >25 residues, respectively, and allow regulatory input. For instance, the C-terminus contains several signals (as indicated) that are required for anterograde and retrograde trafficking (e.g., the interaction with the COPII-dependent vesicular machinery [88,90], and e.g., the FREKLAYAIA motif in the dopamine transporter DAT [89], respectively). Furthermore, transport proteins undergo extensive regulation by post-translational modifications – phosphorylation, ubiquitylation, and palmitoylation (as indicated by serines undergoing phosphorylation marked ‘S’, ubiquitylation sites marked ‘Ub’, and a palmitoylation site marked ‘Pal’). αCaMKII also directly attaches to the C-terminus of DAT and phosphorylates the N-terminus (indicated as CaMK). Membrane-bound proteins that interact with DAT are the SNARE protein syntaxin 1A (in yellow, abbreviated Syn1A) and flotillin 1 (abbreviated as Flot1). The interaction between the DAT-N-terminus and the membrane-bound phosphoinositides is depicted with the two-headed arrows in red and PIP₂. Abbreviations: CaMKII, Ca²⁺/calmodulin-dependent protein kinase II; COPII, vesicle coat protein II; DAT, dopamine transporter; PIP₂, phosphatidylinositol-4,5-bisphosphate; SLC, solute carrier protein; SNARE, soluble NSF attachment protein receptor.

Figure 1

Schematic of the transport cycle. A kinetic scheme of substrate (S) interaction with either the outward- (o) or inward- (i) facing transporter (T) and with cotransported substrates. Substrates can be either physiological substrates such as monoamines or amphetamines; co-substrates can be sodium (Na⁺) or chloride (Cl⁻). There are several additional reactions: in other words, sequential binding of Na⁺ ions, chloride, and substrate to the outward-facing conformation, and the corresponding release steps from the inward-facing conformation ([27] for a more detailed model). These have been omitted for the sake of clarity. The transporter switches from the outward- to the inward-facing conformation via an occluded state (occ). In the serotonin transporter (SERT) the return through the occluded empty state is contingent on binding of K⁺ (symbolized by K⁺ in brackets). The conducting state (Cond) is achieved via an inward-facing conformation [27]. In the presence of amphetamine, amply supplied internal substrate and elevated internal sodium, the transporter releases substrate by running backwards through the cycle (reaction pathway indicated by red arrows). Burst-like dopamine (DA) effluxing events have been recorded by Kahlig and colleagues [47], but their relation to the transport cycle is unclear (indicated by the dashed arrow on the left-hand side).

Figure 2

The molecular effects of amphetamines. Schematic illustration of the effects of amphetamines (AMPH, light-green circles) on the reverse operation of neurotransmitter:sodium symporters (NSS). NSS are present in the plasma membrane either as monomers or oligomers [46]. They are physically linked to the vesicles, and this allows their efficient refilling with monoamines (MA, yellow circles) [7]. The oligomer-based counter-transport model [50] is shown on the left side of the figure and illustrates that the effect of amphetamine relies, at least in part, on an intact oligomer. Amphetamines target the vesicular monoamine transporter (VMAT) and lead to either inhibition and/or reversal of the transport direction to increase the cytosolic concentration of MA, thereby enabling reverse transport. Furthermore, amphetamines inhibit enzymes such as monoamine oxidases A and B (MAO) and thereby prevent the degradation of MA. Abbreviation: TAR1, trace amine receptor 1.